The present invention is generally related to electronic amplifiers, and more specifically to a novel linearization technique applicable to power amplifiers.
Radio frequency (RF) and microwave radios are in common use today. As the number of users increases and the amount of information to be conveyed increases, there is an ever present demand to increase capacity with less available spectrum bandwidth. An obstacle to meeting this demand is distortion introduced into the transmitted signal by the power amplification stage of the radio. For example, one technique used to transmit more data within a given bandwidth is to increase the modulation level. However, unless the transmission path is linear for this increased level of modulation, distortion will be introduced into the transmitted signal. Attempts to reduce distortion have implemented feed forward linearization circuitry to improve the linearity of the power amplifier. A typical feed forward linearization circuit attempts to subtract out distortion by combining a signal having distortion with another signal having a delayed version of the distortion. Typically, this is accomplished by subtracting samples of the input signal from the output signal to produce a sample of the power amplifier""s distortion, and subtracting the amplified sampled distortion from a delayed signal to reduce the distortion in the output signal. The delayed signals are typically created by delay lines in the circuit. However, these delay lines impose restrictions on the bandwidth and the efficiency of the amplifier. An improved linearization scheme is desired.
In one embodiment, an apparatus for reducing amplifier distortion includes an amplifying portion, an error generator portion, and a combining portion. The amplifying portion is configured to receive an input signal or a replica signal, which is an approximate replica of the input signal. The amplifying portion is also configured to provide an amplifier signal comprising an amplifier distortion component. The error generator portion is configured to receive the input signal or the replica signal and provide an error signal comprising an error signal distortion component, which is an approximate replica of the amplifier distortion component. The combining portion is configured to subtract the error signal from the amplifier signal.
In another embodiment a method for reducing amplifier distortion includes amplifying an input signal or an approximate replica of the input signal for generating a first signal comprising an amplifier distortion component. The method also includes amplifying the input signal or the approximate replica of the input signal for generating a second signal comprising an approximate replica of the amplifier distortion component. The method further includes amplifying the input signal or the approximate replica of the input signal for generating an approximately distortionless third signal. Also include are combining the second and third signals for generating a fourth signal indicative of a difference between the second and third signals, and combining the first and fourth signals for subtracting the amplifier distortion component from the first signal.
In yet another embodiment, an amplifier circuit includes a first amplifier electromagnetically coupled to an input terminal and a first coupler. The first coupler is electromagnetically coupled to an output terminal. A second amplifier is electromagnetically coupled to the input terminal and a second coupler. A third amplifier is electromagnetically coupled to the input terminal and the second coupler, and the second coupler is electromagnetically coupled to the first coupler.